Optokinetic functions and optokinetic after-nystagmus-producing mechanism in the monkey.

Optokinetic nystagmus (OKN) and after-nystagmus (OKAN) were investigated in monkeys subjected to conventional and reversed optokinetic stimulation. The decreasing patterns of OKAN slow phase speed (SPS) changed from a straight to a catenary-like pattern with increased stimulus speed. The total duration, however, of the first and second phases remained unchanged. Immediately after a sudden reversal of the stimulus direction, neither OKAN nor OKN was induced; the slow rising portion of OKN SPS became 1.2-1.6 times longer than that of the usual OKN. Since OKN inhibition after stimulation reversal shared similar characteristics with the slow rise of OKN SPS in the light and with the decrease of OKAN SPS in the dark, all of these can be regarded as adaptive processes of the after-nystagmus-producing mechanism.